A semiconductor structure includes substrate, semiconductor layers, source/drain features, metal oxide layers, and a gate structure. The semiconductor layers extend in an X-direction and over the substrate. The semiconductor layers are spaced apart from each other in a Z-direction. The source/drain features are on opposite sides of the semiconductor layers in the X-direction. The metal oxide layers cover bottom surfaces of the semiconductor layers. The gate structure wraps around the semiconductor layers and the metal oxide layers. The metal oxide layers are in contact with the gate structure.